Reduce some nesting in cstruct_to_rs.
Reduce some nesting in cstruct_to_rs.

emilio/rust-bindgen,jdub/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,catdesk/rust-bindgen,emilio/rust-bindgen,jdub/rust-bindgen,catdesk/rust-bindgen,emilio/rust-bindgen,catdesk/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,catdesk/rust-bindgen,jdub/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen